Telephone switching systems typically use a digital port circuit to connect to the communication line from each digital terminal. Since large switching systems can interface many digital terminals, the cost of these digital port circuits can become an appreciable percentage of the cost of the system. The receiver circuit part of the port circuit includes a bulky bifilar wound line transformer to provide dc isolation and longitudinal suppression. This transformer is expensive compared to the cost of other components of the circuit. Moreover, a transformer does not permit transmission of "DC" signal states thereby limiting the types of digital data signals which can be used. Additionally, the physical size of this transformer affects the number of components which can be placed on a port board, the spacing between boards and hence affects the physical size of the system. What is desired is a port circuit design which is smaller and more cost effective.